It bothers me too, but I do understand that heejae is the type of person to feel that he was giving them a gift by allowing them to play his composed piece. So, the fact that it wasn’t a unique experience is acceptable to him since he still saw both of them as special and apart from all other people in his eyes.
I’m not sure if it was ever specified if they were physically intimate or not, but either way that’s why I said lover/relationship. Even if it was a different relationship than lovers, it was an intimate sharing between people that garam believes was uniquely shared between he and heejae, but the same/very similar experience was also shared between heejae and his manager (which garam is unaware of)

Heejae presenting the same music to Garam that heejae shared with a previous lover/relationship is a bit upsetting. I do understand why he did it. The previous manager and Garam are the only two people he felt so connected to that he wanted them to play his composed piece. I get it, but it still makes me feel sad for Garam, because to Garam it was an experience he thought they were uniquely sharing, especially since Heejae worded it as “you’re the first student I’ve shown this to.” Technically he didn’t lie, since he showed a previous manager not another student, but it still feels like when a person takes their new partner to the same “special” place/date that they shared with their ex. Even though it wasn’t meant to be deceptive or hurtful, it still feels a bit sad to know Garam wasn’t actually the first important person the professor shared his music with.